Process for the absorption of nitrogen oxides from gases



M. vooGD 2,110,431 PROCESS FOR THE ABSORPTION OF NITROGEN OXIDES FROM GASES n March8, 1938.

Filed Aug. 3, 1934 Flr KWK SS IIII [NVE/wop: MAARrE/v Vo o natural sodium nitrate, Whilst the yield is Patented Mar. 8, 1938 UNITED STATES PROCESS FOR THE ABSORPTION OF NITRO- GEN OXIDES FROM GASES Maarten Voogd, Beverwijk, Netherlands, assignor to Shell Development Company, San Francisco, Calif., a corporation of Delaware Application In the 4 C'laims.

The invention relates to a process for the absorption of nitrogen oxides, from gases containing these oxides and, if desired, for manufacturing ammonia salts.

In general gases containing nitrogen oxides, particularly the gases leaving the acid absorption in the manufacture of nitric acid from oxidation products of ammonia, are to be taken into consideration as base material. The latter gases consist for the greater part of nitrogen and contain in addition a few percent of oxygen and a usually very small amount of nitrogen oxides.

In order to prevent the spreading of poisonous gases in the surroundings and further to convert the nitrogen oxides contained in the gases mentioned above into useful products, an alkaline absorption is usually carried out after the acid absorption. When the gases flow through this alkaline absorption, consisting of one or more absorption towers in which a soda solution of about concentration circulates, the nitrogen oxides are bound in the form of a mixture of sodium nitrate and sodium nitrite. As soon as the soda has been converted practically entirely a fresh solution of soda is taken into use. The sodium nitrite and sodium nitrate is then treated with nitric acid, the nitrous acid set free blown out with steam and the remaining acid solution of sodium nitrate, after having been neutralized with soda, worked up to solid sodium nitrate.

A great drawback of this soda absorption is that it is very expensive, so that the sodium nitrate obtained is considerably dearer than the only small.

It has now been found that a practically complete absorption of the nitrogen oxides can be achieved in a particularly advantageous manner whilst avoiding the expensive soda absorption and the small production of sodium nitrate by bringing the gases containing the nitrogen oxides whilst in the presence of water, water vapour, or an aqueous medium and of oxygen or a gas containing or yielding oxygen, into contact with NH3. Preferably this NH3 is previously added to the gas, but the gas may also be led into ammoniacal water. If so desired both measures may be applied.

When using aqueous solutions of ammonia for the absorption a certain loss of ammonia occurs owing to the high vapour tension of the solutions in question when the gases flow through the absorption towers in which the solution of ammonia circulates.

With the process according to the inventionv gaseous ammonia is preferably added to the gases in View of this loss, after which the gases containing ammonium nitrite in the form of mist are brought into contact with water or an August 3, Netherlands August 11, 1933 1934, Serial No. 738,373

aqueous medium. In any loss of ammonia.

It is recommendable to add a slight excess of NH3 in proportion to the nitrogen oxides contained Vin the gases, so that the absorption liquid is just ammoniacal.

this manner there is hardly Thus the corrosion of iron is prevented, whilst the solution of nitrite obtained becomes more stable.

The process according to the invention may, for instance, be carried out as follows:

A very slight excess of ammonia gas, calculated on the nitrous gases present, is admixed to the gas leaving the acid absorption in the manufacture of nitric acid from the oxidation products of ammonia and having the following composition: about 97.8% N2, about 2% O2 and about 0.2% nitrous gases. The ammonia gas reacts with the nitrogen oxides, the oxygen and the water Vapour contained in the gas, whilstat the same time a mist of ammonium nitrate and ammonium nitrite is formed. Then the gases containing nitrate. and nitrite are brought into contact with water or an aqueous salt solution, e. g. aqueous ammonium nitrate in one or more absorption towers. 'I'he mists formed dissolve in the water, whilst the nitrogen oxides which sc far have not been converted react further to ammonium nitrite and nitrate. The rate of flow of the gas is adjusted so that the gas on leaving the absorption towers practically nolonger contains any nitrogen oxides. The reaction takes place Very rapidly, so that even when the time of contact is very short the residual gas contains only a few hundredths percent of nitrogen oxides.

The solution resulting from the absorption and containing principally ammonium'nitrite in addition to more or less considerable quantities 'of ammonium nitrate may be circulated in the absorption towers until the concentration of the nitrite is sufficient, after which the solution may be removed from the system either continuously or intermittently and the nitrite may be converted into the corresponding ammonia salts in a known manner by treating it with strong acids, such as nitric acid, sulphuric acid, hydrochloric acid, etc. and blowing out the liberated nitrogen oxides with air, oxygen or air enriched with oxygen or some other gas. The nitrogen oxides hereby set free are returned to the acid absorption, whilst the solution of ammonia salt obtained, if circumstances permit, is worked up to solid salt.

If the nitric acid resulting from the acid absorption is to be used for manufacturing ammonium nitrate, it is of particular advantage to add the solution of ammonium nitrite resulting from the nal absorption according to the invention to the nitric acid during or before the last manipulation of this acid, Viz. blowing out With air (the so-called bleaching) Y o generally carried out in a small tower, the slightlyY Y vpreheatedacid trickling down the filling material Y bleaching vessel. are then returned This bleaching process is of the tower countercurrently to a stream'of air or oxygen or other gases.

acid is'introduced into The liberatednitrogen oxides toV the acid absorption whilst the nitric acid containing ammonium nitrate obtainedis workedrup to pure ammonium-nitrate V nium nitrite into composition readily occurs inan acid medium. SA.V Suitable manner to perform this consists inV passing the oxygen Vor the air or mixtures of such Y or mixtures containing ammoniumfnitratarsuch i as vcalcium Anitrate containing ammonium n itrate, double salts of ammonium nitrate and sulphate, mixtures of ammonium nitrate. with calcium carbonate and therlike. Y Y

:Both during the separate conversion of the ammonium nitrite solution with strong acids,A such.

as nitric. acid, Ysulph'uric acid, hydrochloricr acid, into the corresponding ammonium salts Awhilst removing, the liberated nitrogen .oxides :as described above and during such conversion inV combination with the last manipulation to be applied to crude nitric a'cid, torwit the blowing out withi air` (the so-called bleaching; referred to in the preceding paragraph) it is recommended to carry out this blowing-outV operation as quickly as possible, in order to avoid decomposition of' ammonitrogen and Water, which degases in very small bubbles through the liquid to be treated. The known gas distributers, such as porous stones, perforated or porous stone-ware I pipes or-thevlike, may be used-for this 'purpose Y Inthis manner the'rea'ction 4Q4 2 I" j is? avoided as much as possible,whilst the reaction V Y f v 2NH4Nog+2HN03 2NH4NO3+NO+N02+H20 'L is promoted. o o Y K i I claim as myY invention: v v v1. Process Vforthe absorption of nitrogenoxi-Y a solution -of the the top Vof'a` n nitricacidsinto the 'top of said tower, forcing air vffof crude ynitric acid stream whereby a mixture ofV ammoniumnitrate ides from gases leaving the acid absorption sys- -tem vof a nitric acid plantcomprising! contacting said gases with a concurrently flowing stream of ammonia gasV in excess while in the presence oi' Instead of a bleach-- ing tower a bleaching vessel may be= used. wThe solution ofammonium nitriteY andY nitrate,` Atolgether with theifreshly produced crude nitric K the bleaching tower or oxygen and water vapor and subsequently forming resulting ammonium salts in a solventetherefor, passing the resulting solution of ammoniumfnitrite and ammonium nitrate into tower simultaneously passing crude into the bottom of'Y said tower countercurrent to said introducedv liquids, and withdrawing the resulting nitric acid-ammonium nitrate solution from` the bottom of said tower.

2. Process for the absorptionV of Vnitrogen oxidesfrom gases leaving the acid absorption system ofa nitric acid plant comprising contacting the tracesY of nitrogen oxides with Ysuiicient gaseous ammonia in excess to neutralize the `oxides of'` nitrogen in said gas'stream and render the resulting stream slightly alkaline, passing saldresulting gas stream into 'contact with a lsolution of ammonium nitrate', whereby amounts of ammoniumnitrateV and ammonium nitrite solutionY are formed, passing said ammonium nitrite and ammonium nitrate, solution together withamounts countercurrentY to an air and nitric acid. is produced. Y 3. Process for absorbing traces of nitrogen x- Yides from the vgas stream Yleaving the acid absorptionsystem of a nitric acid plant comprising continuously first contacting said'traces of nitrogen oxides Withea concurrently owing stream of free g ammonia in excessfwhile' in the presence of wa- -V ter vapor and air and then removing as a solution the ammonium formed.;

f V4. In aprocess for the nitrite and ammonium nitrate production of nitric acid 't by absorbing nitrogen oxides in dilute nitric acid,

the steps of mixingthe taining about .2%

residual gas streamcon- Whilein the presence ofofwater vapor and air and thereafter removing asV a. solution the ammonium nitrite and ammonium nitrate formed.

nitrogen `oxides with a concur- ,rently .flowing stream'of ammonia gas in excess 

